How close are we to having an AI that can play Magic: The Gathering objectively well?
This is a very specific task, with clearly defined parameters, so it would already theoretically be within the scope of current AI technology to do this.

The AI would need to learn how to make decisions, and the best way to do this is the approach taken to teaching an AI to play Go - seeing thousands of example games by experts, and playing itself thousands of times.

The AI won't necessarily "understand" what a card represents, the way a human would, but it can learn to make the same kinds of decisions as the human would.

The difficulties would be purely practical - Go is very easy to represent digitally, because the options for action are limited to placing a stone on one of the intersections, and there is only one opponent. Magic is more complex, so the developers would need to spend sufficient resources to be quite sure they had captured all the relevant variables in the digital representation. Then, of course, they would need to encode thousands of games of Magic (or deck-building processes) into that digital format, so the AI could study them.